Temperature dependence on the electrical properties of Ba(Ti0.90Zr0.10)O-3:2V ceramics

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

An N2: CH4: H2O DC glow discharge plasma probed by optical and electric techniques: significance to the radiation chemistry of Titan's upper atmosphere in the presence of meteoritic water

This paper presents optical and electrical measurements on plasma generated by DC excited glow discharges in mixtures composed of 95% N2, 4.8% CH4 and 0.2% H2O at pressures varying from 1.064 mbar to 4.0 mbar. The discharges simulate the chemical reactions that may occur in Titan's atmosphere in the presence of meteorites and ice debris coming from Saturn's systems, assisted by cosmic rays and high energy charged particles. The results obtained from actinometric optical emission spectroscopy, combined with the results from a pulsed Langmuir probe, show that chemical species CH, CN, NH and OH are important precursors in the synthesis of the final solid products and that the chemical kinetics is essentially driven by electronic collision processes. It is shown that the presence of water is sufficient to produce complex solid products whose components are important in prebiotic compound synthesis. © 1998 Elsevier Science Ltd. All rights reserved.

MnO2 influence on the electrical properties of SnO2-based ceramic systems

Tin dioxide is an n-type semiconductor that when doped with other metallic oxides exhibits non-linear electric behavior with high non-linear coefficient values typical of a varistor. In this work, electrical properties of the SnO2.CoO.Ta2O5 and SnO2.CoO.MnO2.Ta2O5 ceramics systems were studied with the objective of analyzing the influence of MnO2 on sintering behavior and electrical properties of these systems. The compacts were prepared by powder mixture process and sintered at 1300°C for 1 hour, in air, using a constant heating rate of 10°C/min. The morphological and structural properties were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The densities of the sintered ceramics were measured using the Archimedes method. The SnO2.CoO.Ta2O5 and SnO2.CoO.MnO2.Ta2O5 systems presented breakdown fields (Eb) about 3100 V.cm-1 and 3800 V.cm-1, respectively, and non-linear coefficient (α) about 10 and 20, respectively.

Structural and dielectric properties of polyvinyl alcohol/barium zirconium titanate polymer-ceramic composite

The polyvinyl alcohol (PVA)/barium zirconium titanate Ba[Zr0.1Ti0.9]O3 (BZT) polymer-ceramic composites with different volume percentage are obtained from solution mixing and hot-pressing method. Their structural and electrical properties are characterized by X-ray diffraction (XRD), Rietveld refinement, cluster modeling, scanning electron microscope and dielectric study. XRD patterns of PVA/BZT polymer-ceramics composite (with 50% volume fractions) indicate no obvious differences than the XRD patterns of pure BZT which shows that the crystal structure is still stable in the composite. The scanning electron micrograph indicates that the BZT ceramic is dispersed homogeneously in the polymer matrix without agglomeration. The dielectric permittivity (ε r) and the dielectric loss (tan δ) of the composites increase with the increase of the volume fraction of BZT ceramic. Theoretical models are employed to rationalize the dielectric behavior of the polymer composites. The dielectric properties of the composites display good stability within a wide range of temperature and frequency. The excellent dielectric properties of these polymer-ceramic composites indicate that the BZT/PVA composites can be a candidate for embedded capacitors. © 2013 Elsevier B.V.

Microstructural and Nonohmic Properties of ZnO center dot Pr6O11 CoO Polycrystalline System

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Growth and Characterization of Silicon Carbide Thin Films Using a Nontraditional Hollow Cathode Sputtering Technique

Silicon carbide (SiC) is considered a suitable candidate for high-power, high-frequency devices due to its wide bandgap, high breakdown field, and high electron mobility. It also has the unique ability to synthesize graphene on its surface by subliming Si during an annealing stage. The deposition of SiC is most often carried out using chemical vapor deposition (CVD) techniques, but little research has been explored with respect to the sputtering of SiC. Investigations of the thin film depositions of SiC from pulse sputtering a hollow cathode SiC target are presented. Although there are many different polytypes of SiC, techniques are discussed that were used to identify the film polytype on both 4H-SiC substrates and Si substrates. Results are presented about the ability to incorporate Ge into the growing SiC films for the purpose of creating a possible heterojunction device with pure SiC. Efforts to synthesize graphene on these films are introduced and reasons for the inability to create it are discussed. Analysis mainly includes crystallographic and morphological studies about the deposited films and their quality using x-ray diffraction (XRD), reflection high energy electron diffraction (RHEED), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), Auger electron spectroscopy (AES) and Raman spectroscopy. Optical and electrical properties are also discussed via ellipsometric modeling and resistivity measurements. The general interpretation of these analytical experiments indicates that the films are not single crystal. However, the majority of the films, which proved to be the 3C-SiC polytype, were grown in a highly ordered and highly textured manner on both (111) and (110) Si substrates.

DNA - novel nanomaterial for applications in photonics and in electronics

Functionalization with surfactants and with active molecules of deoxyribonucleic acid (DNA), thin film processing as well as their nonlinear optical and electrical properties are reviewed and discussed. On the basis of a quantum three level model, we show that the anomalous concentration variation of cubic susceptibility chi((3))(-3 omega; omega, omega, omega) in thin films of DNA-CTMA complexes doped with Disperse Red 1 chromophore can be explained by the concentration variation of two-photon resonance contribution. We show also that the DNA complexes, plasticized with glycerol and adequately doped can be processed into self standing conducting membranes with a high electrical conductivity. The measured ionic conductivity at room temperature, depending on dopant used and its concentration, is in the range of 3.5 x 10(-4)-10(-5) S/cm and increases linearly as a function of temperature, reaching 10(-3) S/cm at 358 K for the most conducting sample, obeying predominantly the Arrhenius law. Practical applications of DNA complexes are also described and discussed. (C) 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

Advanced schemes for surface plasmon resonance and plasmon-enhanced fluorescence biosensors

Advanced optical biosensor platforms exploiting long range surface plasmons (LRSPs) and responsive N-isopropylacrylamide (NIPAAm) hydrogel binding matrix for the detection of protein and bacterial pathogen analytes were carried out. LRSPs are optical waves that originate from coupling of surface plasmons on the opposite sites of a thin metallic film embedded between two dielectrics with similar refractive indices. LRSPs exhibit orders of magnitude lower damping and more extended profile of field compared to regular surface plasmons (SPs). Their excitation is accompanied with narrow resonance and provides stronger enhancement of electromagnetic field intensity that can advance the sensitivity of surface plasmon resonance (SPR) and surface plasmon-enhanced fluorescence spectroscopy (SPFS) biosensors. Firstly, we investigated thin gold layers deposited on fluoropolymer surface for the excitation of LRSPs. The study indicates that the morphological, optical and electrical properties of gold film can be changed by the surface energy of fluoropolymer and affect the performance of a SPFS biosensor. A photo-crosslinkable NIPAAm hydrogel was grafted to the sensor surface in order to serve as a binding matrix. It was modified with bio-recognition elements (BREs) via amine coupling chemistry and offered the advantage of large binding capacity, stimuli responsive properties and good biocompatibility. Through experimental observations supported by numerical simulations describing diffusion mass transfer and affinity binding of target molecules in the hydrogel, the hydrogel binding matrix thickness, concentration of BREs and the profile of the probing evanescent field was optimized. Hydrogel with a up to micrometer thickness was shown to support additional hydrogel optical waveguide (HOW) mode which was employed for probing affinity binding events in the gel by means of refractometric and fluorescence measurements. These schemes allow to reach limits of detection (LODs) at picomolar and femtomolar levels, respectively. Besides hydrogel based experiments for detection of molecular analytes, long range surface plasmon-enhanced fluorescence spectroscopy (LRSP-FS) was employed for detection of bacterial pathogens. The influence of capture efficiency of bacteria on surfaces and the profile of the probing field on sensor response were investigated. The potential of LRSP-FS with extended evanescent field is demonstrated for detection of pathogenic E. coli O157:H7 on sandwich immunoassays . LOD as low as 6 cfu mL-1 with a detection time of 40 minutes was achieved.rn

Electronic structure and physical properties of Heusler compounds for thermoelectric and spintronic applications

This thesis focuses on synthesis as well as investigations of the electronic structure and properties of Heusler compounds for spintronic and thermoelectric applications.rnThe first part reports on the electronic and crystal structure as well as the mechanical, magnetic, and transport properties of the polycrystalline Heusler compound Co2MnGe. The crystalline structure was examined in detail by extended X-ray absorption fine structure spectroscopy and anomalous X-ray diffraction. The low-temperature magnetic moment agrees well with the Slater-Pauling rule and indicates a half-metallic ferromagnetic state of the compound, as is predicted by ab-initio calculations. Transport measurements and hard X-ray photoelectron spectroscopy (HAXPES) were performed to explain the electronic structure of the compound.rnA major part of the thesis deals with a systematical investigation of Heusler compounds for thermoelectric applications. Few studies have been reported on thermoelectric properties of p-type Heusler compounds. Therefore, this thesis focuses on the search for new p-type Heusler compounds with high thermoelectric efficiency. The substitutional series NiTi1−xMxSn and CoTi1−xMxSb (where M = Sc, V and 0 ≤ x ≤ 0.2) were synthesized and investigated theoretically and experimentally with respect to electronic structure and transport properties. The results show the possibility to create n-type and p-type thermoelectrics within one Heusler compound. The pure compounds showed n-type behavior, while under Sc substitution the system switched to p-type behavior. A maximum Seebeck coefficient of +230 μV/K (at 350 K) was obtained for NiTi0.26Sc0.04Zr0.35Hf0.35Sn, which is one of the highest values for p-type thermoelectric compounds based on Heusler alloys up to now. HAXPES valence band measurement show massive in gap states for the parent compounds NiTiSn, CoTiSb and NiTi0.3Zr0.35Hf0.35Sn. This proves that the electronic states close to the Fermi energy play a key role for the behavior of the transport properties. Furthermore, the electronic structure of the gapless Heusler compounds PtYSb, PtLaBi and PtLuSb were investigated by bulk sensitive HAXPES. The linear behavior of the spectra close to εF proves the bulk origin of Dirac-cone type density of states. Furthermore, a systematic study on the optical and transport properties of PtYSb is presented. The compound exhibits promising thermoelectric properties with a high figure of merit (ZT = 0.2) and a Hall mobility μh of 300 cm2/Vs at 350 K.rnThe last part of this thesis describes the linear dichroism in angular-resolved photoemission from the valence band of NiTi0.9Sc0.1Sn and NiMnSb. High resolution photoelectron spectroscopy was performed with an excitation energy of hν = 7.938 keV. The linear polarization of the photons was changed using an in-vacuum diamond phase retarder. Noticeable linear dichroism is found in the valence bands and this allows for a symmetry analysis of the contributing states. The differences in the spectra are found to be caused by symmetry dependent angular asymmetry parameters, and these occur even in polycrystalline samples without preferential crystallographic orientation.rnIn summary, Heusler compounds with 1:1:1 and 2:1:1 stoichiometry were synthesized and examined by chemical and physical methods. Overall, this thesis shows that the combination of first-principle calculations, transport measurements and high resolution high energy photoelectron spectroscopy analysis is a very powerful tool for the design and development of new materials for a wide range of applications from spintronic applications to thermoelectric applications.rn

Substrate nanopatterning by e-beam lithography to growth ordered arrays of III-nitride nanodetectors, white light emitters, and solar cells

III-nitride nanorods have attracted much scientific interest during the last decade because of their unique optical and electrical properties [1,2]. The high crystal quality and the absence of extended defects make them ideal candidates for the fabrication of high efficiency opto-electronic devices such as nano-photodetectors, light-emitting diodes, and solar cells [1-3]. Nitride nanorods are commonly grown in the self-assembled mode by plasma-assisted molecular beam epitaxy (MBE) [4]. However, self-assembled nanorods are characterized by inhomogeneous heights and diameters, which render the device processing very difficult and negatively affect the electronic transport properties of the final device. For this reason, the selective area growth (SAG) mode has been proposed, where the nanorods preferentially grow with high order on pre-defined sites on a pre-patterned substrate

Lattice Engineering of III-Nitride Heterostructures and Their Applications

III-Nitride materials have recently become a promising candidate for superior applications over the current technologies. However, certain issues such as lack of native substrates, and high defect density have to be overcome for further development of III-Nitride technology. This work presents research on lattice engineering of III-Nitride materials, and the structural, optical, and electrical properties of its alloys, in order to approach the ideal material for various applications. We demonstrated the non-destructive and quantitative characterization of composition modulated nanostructure in InAlN thin films with X-ray diffraction. We found the development of the nanostructure depends on growth temperature, and the composition modulation has impacts on carrier recombination dynamics. We also showed that the controlled relaxation of a very thin AlN buffer (20 ~ 30 nm) or a graded composition InGaN buffer can significantly reduce the defect density of a subsequent epitaxial layer. Finally, we synthesized an InAlGaN thin films and a multi-quantum-well structure. Significant emission enhancement in the UVB range (280 – 320 nm) was observed compared to AlGaN thin films. The nature of the enhancement was investigated experimentally and numerically, suggesting carrier confinement in the In localization centers.

Structural and Electrical Investigation of C 60 –Graphene Vertical Heterostructures

Graphene, with its unique electronic and structural qualities, has become an important playground for studying adsorption and assembly of various materials including organic molecules. Moreover, organic/graphene vertical structures assembled by van der Waals interaction have potential for multifunctional device applications. Here, we investigate structural and electrical properties of vertical heterostructures composed of C60 thin film on graphene. The assembled film structure of C60 on graphene is investigated using transmission electron microscopy, which reveals a uniform morphology of C60 film on graphene with a grain size as large as 500 nm. The strong epitaxial relations between C60 crystal and graphene lattice directions are found, and van der Waals ab initio calculations support the observed phenomena. Moreover, using C60-graphene heterostructures, we fabricate vertical graphene transistors incorporating n-type organic semiconducting materials with an on/off ratio above 3 × 10(3). Our work demonstrates that graphene can serve as an excellent substrate for assembly of molecules, and attained organic/graphene heterostructures have great potential for electronics applications.

Aluminum nanoholes for optical biosensing

Sub-wavelength diameter holes in thin metal layers can exhibit remarkable optical features that make them highly suitable for (bio)sensing applications. Either as efficient light scattering centers for surface plasmon excitation or metal-clad optical waveguides, they are able to form strongly localized optical fields that can effectively interact with biomolecules and/or nanoparticles on the nanoscale. As the metal of choice, aluminum exhibits good optical and electrical properties, is easy to manufacture and process and, unlike gold and silver, its low cost makes it very promising for commercial applications. However, aluminum has been scarcely used for biosensing purposes due to corrosion and pitting issues. In this short review, we show our recent achievements on aluminum nanohole platforms for (bio)sensing. These include a method to circumvent aluminum degradation—which has been successfully applied to the demonstration of aluminum nanohole array (NHA) immunosensors based on both, glass and polycarbonate compact discs supports—the use of aluminum nanoholes operating as optical waveguides for synthesizing submicron-sized molecularly imprinted polymers by local photopolymerization, and a technique for fabricating transferable aluminum NHAs onto flexible pressure-sensitive adhesive tapes, which could facilitate the development of a wearable technology based on aluminum NHAs.

Hall Effect and transient surface photovoltage (SPV) study of Cu3BiS3 thin films

Se presentan las propiedades eléctricas del compuesto Cu3BiS3 depositado por co-evaporación. Este es un nuevo compuesto que puede tener propiedades adecuadas para ser utilizado como capa absorbente en celdas solares. Las muestras fueron caracterizadas a través de medidas de efecto Hall y fotovoltaje superficial transiente (SPV). A través de medidas de efecto Hall se encontró que la concentración de portadores de carga n es del orden de 1016 cm-3 independiente de la relación de masas de Cu/Bi. También se encontró que la movilidad de este compuesto (μ del orden de 4 cm2V -1s-1) varía de acuerdo con los mecanismos de transporte que la gobiernan en dependencia con la temperatura. A partir de las medidas de SPV se encontró alta densidad de defectos superficiales, defectos que son pasivados al superponer una capa buffer sobre el compuesto Cu3BiS3.

Optimized multilayer indium-free electrodes for organic photovoltaics

Flexible multilayer electrodes that combine high transparency, high conductivity, and efficient charge extraction have been deposited, characterised and used as the anode in organic solar cells. The anode consists of an AZO/Ag/AZO stack plus a very thin oxide interlayer whose ionization potential is fine-tuned by manipulating its gap state density to optimise charge transfer with the bulk heterojunction active layer consisting of poly(n-3- hexylthiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester (P3HT:BC61BM). The deposition method for the stack was compatible with the low temperatures required for polymer substrates. Optimisation of the electrode stack was achieved by modelling the optical and electrical properties of the device and a power conversion efficiency of 2.9% under AM1.5 illumination compared to 3.0% with an ITO-only anode and 3.5% for an ITO:PEDOT electrode. Dark I-V reverse bias characteristics indicate very low densities of occupied buffer states close to the HOMO level of the hole conductor, despite observed ionization potential being high enough. Their elimination should raise efficiency to that with ITO:PEDOT.